1. Field of the Invention
The invention is in the general field of communications protocols and methods, more specifically in wireless communications protocols and methods.
2. Description of the Related Art
As wireless communications technology has advanced, the general problem of how to crowd more and more users, each wanting to send and receive more and more amounts of data onto limited regions of the available wireless radio spectrum has increased.
Today, there are a huge number of different portable hand-held devices with wireless capability in use. These portable wireless devices (such as cell phones, portable computers, and the like) are often powered by small batteries, and the users typically expect these devices to operate for many hours before the batteries are recharged. To meet these user expectations, the wireless transmitters on these devices must output wireless signals using very small amounts of power, making it difficult to distinguish the wireless radio signal over background noise.
An additional problem is that many of these devices are carried on moving vehicles, such as automobiles, airplanes, and the like. This causes additional complications because the low-power wireless signal transmitted by these devices can also be subjected to various distortions, such as varying and unpredictable Doppler shifts, and unpredictable multi-path effects often caused by varying radio reflections off of buildings or other structures.
And against all these problems, the noise background of the various wireless channels becomes ever higher as noise-producing electrical devices proliferate. The proliferation of other wireless devices also adds to the background noise.
In order to cope with these problems, a number of prior art radio transmission methods, including Time Division Multiple Access (TDMA), the Global System for Mobile Communications (GSM), Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency-Division Multiplexing (OFDM), and other wireless protocols have become widespread. Each attempts to solve the above set of problems in a slightly different way.
One of the simplest, yet highly popular, wireless communications schemes is TDMA, In TDMA, a single carrier frequency can be shared with multiple users, and each user is assigned their own particular time-slot, which in turn is interleaved with the time slots reserved for other users.
TDMA forms the basis for the more advanced GSM method, which combines time hopping with frequency hopping, and is the most popular mobile phone protocol in the world today. GSM allocates a number of different frequencies or wavelengths for use, and each wavelength is divided into small timeslots often only a few milliseconds in duration. Thus each wavelength can be shared with about eight to sixteen other users.
By contrast, CDMA is more complex protocol. CDMA is a spread-spectrum protocol that assigns each transmitter a different spreading-code. This spreading-code is usually a pseudo-random number or code that runs at a much higher “chip” rate (a chip is typically a very brief rectangular pulse of +1 or −1 amplitude) than the underlying data signal that the user wishes to transmit. CDMA works by modulating the user's data signal with the high speed pseudo-random spreading code by, often by an exclusive OR (XOR) process, before transmitting the result. This high speed spreading-code effectively distributes the user's data across a broader spectrum of wavelengths, and also provides a convenient way for a receiver to “tune” in to the signal and distinguish the signal from background noise.
By using different pseudo-random numbers, a number of different users can use different spreading codes, and all simultaneously transmit on the same frequency band. At the receiving end, a receiver knowing the proper pseudo-random spreading code can “tune in” to the desired transmitter, and ignore the other signals. The effect of the other users transmitting using different spreading codes on the same frequency band is minimal because their different spreading codes only raise the background noise on the channel by a small amount. Indeed, because spreading codes spread the data across a broader spectral range, spread-spectrum methods such as CDMA are considered to be relatively resistant to background noise.
By contrast, the alternative FDMA protocol works by subdividing the available spectrum into a number of narrow band channels. Users are each assigned a unique frequency or wavelength (a unique narrow band) during each communications session.
The OFDM (also called discrete multi-tone modulation or DMT) protocol also divides the available spectrum into many narrow band channels or tone or spectral-shapes, each separated from each other by a minimal frequency or wavelength separation. These narrow band tone or spectral-shape channels can carry quadrature-amplitude modulated (QAM) signals or phase-shift keyed signals designed to minimally overlap (be orthogonal with) with similar signals carried by adjacent narrow band channels. The OFDM scheme essentially maps out a limited time-frequency grid so that each symbol that is transmitted is allocated a particular time and frequency coordinate. Because the OFDM scheme uses very closely packed channels, as well as carrier modulation schemes chosen for minimal overlap between these many closely packed tone or spectral-shape channels, the method can achieve a relatively high rate of data transmission.
Although the OFDM scheme uses available spectrum fairly efficiently, and thus is popular for both wireless and wired wideband digital communications, the method requires very accurate frequency synchronization between the receiver and the transmitter. Thus the OFDM protocol is relatively sensitive to problems caused by Doppler shift, and these problems are made still worse when reflections off of buildings and other structures cause multi-path effects.